1. Field of the Invention
The present invention relates to an optical encoder that can realize absolute position detection, used for displacement measurement or angle measurement.
2. Related Background Art
The optical encoders basically composed of a main scale on which a first optical grating is formed, an index scale opposed to the main scale on which a second optical grating is formed, a light source for illuminating the main scale with light, a light receiving element for receiving light that has been transmitted or reflected by the first optical grating on the main scale and then returned from the second optical grating on the index scale.
In connection with this type of optical encoder, a system using a light receiving element array that also functions as an index scale has been already proposed, for example in Japanese Patent Publication No. H06-56304. The inventors of this patent application also disclosed an optical encoder of a similar type in Japanese Patent Application Laid-Open No. 2003-161645.
Such an encoder is called an incremental type encoder, in which when the scale moves, the movement amount can be detected based on increase and decrease of a pulse. The incremental type encoder suffers from the problem that the absolute position cannot be detected and therefore a separate sensor for detecting the absolute position is required to be equipped.
FIG. 40 is a perspective view showing an optical encoder for detecting the absolute position in an incremental type encoder. This encoder is constructed as a transmissive optical encoder having a light source 1 such as an LED, a collimator lens 2 for converting a light flux from the light source 1 into a parallel light flux, a main scale 3, an index scale 4 and a light receiving portion 5 including a plurality of light receiving elements.
FIG. 41 is a plan view of the main scale 3, in which a plurality of slits S1, S2, . . . , S15, . . . are arranged at regular intervals, and a slit H is provided as a marking opening in order to generate a origin point signal.
FIG. 42 is a plan view of the index scale 4, in which an opening pattern provided for generating incremental A and B phase signals is shown. Windows W1, W2, W3 and W4, each of which has three slit-like openings, are arranged with a spatial mutual phase difference of 90 degree. A window WH is associated with the origin marking opening H of the main scale 3.
FIG. 43 shows a light receiving portion 5, in which photodiodes P1, P2, P3 and P4 are provided for receiving light having been passed through the windows W1, W2, W3 and W4 respectively. A photodiode PH is associated with the origin marking opening H of the main scale 3. When the positions of openings of the main scale 3 and the index scale are aligned, light from the light source passes through them and received by the light receiving element.
FIG. 44 shows statuses a, b, c and d in a process through which the main scale 3 moves relative to the index scale at ¼ pitch displacement steps. The overlapping state of the openings of the index scale and the slits S of the main scale gradually changes through statuses a to d, and the result of the change is detected by the light receiving element 5. FIG. 45 shows the output signals of the photodiodes P1 to P4 and PH in the statuses a to d shown in FIG. 44.
In the conventional arrangement as described above, it is necessary to provide an additional light receiving element designed for detecting the origin marking opening H. This invites an increase in the size of the element. In addition, it is necessary in this origin detection method to provide a light receiving element for generating an original point signal. It also suffers from the problem that the phase relationship relative to the incremental phase is broken by an azimuth displacement of the main scale 3 about the optical axis of the light source 1 as the rotation axis.